1. Field of the Invention
The present invention relates to a laser optical system, and more particularly, to a laser optical system for a liquid crystal display device (LCD), which includes a filter for changing a laser beam profile.
2. Description of the Background Art
A liquid crystal display (LCD) device has been widely used to display images. Since an image display device requires high speed, the LCD has been developed to have a switching device with high speed operational characteristics. Also, the LCD device uses an array substrate on which a plurality of unit pixels are arranged in a matrix form. A thin film transistor (TFT) is generally used as the switching device for driving the unit pixels. The LCD device uses the TFT having a polysilicon channel layer, which has a high electric mobility. In order to fabricate the polysilicon, a heat treatment method or a laser annealing method may be used. The heat treatment method heats an amorphous silicon in a high temperature furnace until the amorphous silicon is crystallized. The laser anneal method irradiates a high density laser beam to the amorphous silicon to crystallize the amorphous silicon. In the laser annealing method, the crystallization may be done below the glass transition temperature (Tg). Since the laser beam is irradiated to the amorphous silicon in a very short time, the crystallization of the amorphous silicon by the laser anneal method may not cause deformation of glass substrate.
In order to crystallize the amorphous silicon by the laser annealing method, it is necessary to homogeneously irradiate the laser beam to a target material, such as amorphous silicon. Accordingly, a laser optical system for generating a homogeneous laser beam has been designed to include a laser generator, an attenuator controlling density of laser energy from the laser generator, a linear beam processing optical system for changing the laser light having the controlled energy intensity from the attenuator into a linear beam, and a condenser lens for condensing the laser beam from the linear beam processing optical system. The laser beam condensed by the condenser lens is irradiated to the target material, thereby crystallizing an amorphous silicon layer on the target material.
FIG. 1 is an exemplary diagram illustrating irradiation of a laser light 20 to a substrate (target material) 10. Referring to FIG. 1, the laser light 20 condensed by a condenser lens 40 is a linear laser beam 30 of bar-type having a length of L0 and a width of W0. During crystallization, the laser beam 30 is moved in the direction of width W0 on the substrate 10 by moving the laser optical system or the substrate 10.
FIG. 2 is a view illustrating a width profile of a laser beam. As shown in FIG. 2, the laser beam's width has a trapezoid shape, but also shows a specific peak in the edges of the width. These peak edges cause defects in the crystallization of the target material, which will now be explained with reference to FIG. 3.
FIG. 3 is a graph showing a relationship between laser intensity and crystal grain size. As shown in FIG. 3, the size of grains and the intensity of the irradiated laser energy are proportional to each other until a predetermined energy intensity “Ec,” which is regarded as a complete melting point. However, the grain size sharply decreases over the predetermined energy intensity “Ec,” and small grains of about 100 (nm) are formed. Specifically, when the laser beam is irradiated to the amorphous silicon layer, the surface of the amorphous silicon is directly exposed by the laser beam, thereby melting the amorphous silicon. However, since the lower portion of the amorphous silicon layer is irradiated by a weak laser beam, some non-melted silicon particles may remain. Therefore, large grains are grown from seeds of the lower portion of the amorphous silicon during the crystallizing.
On the other hand, when the laser beam having an intensity exceeding the complete melting point is irradiated to the amorphous silicon, the amorphous silicon does not have seeds of crystallization. When the completely melted amorphous silicon is cooled without the seeds, the seeds are randomly generated in a melted region, and the amorphous silicon is crystallized from the seeds while the grain size is very small. A point “B” of FIG. 3 shows the small grain size formed in a completely melted region.
Accordingly, when the laser beam is irradiated to the amorphous silicon in the width direction, if the peak is generated in the edges of the laser as shown in FIG. 2, the grains generated by the peak laser are very small. That is, as shown in FIG. 3, when the laser beam having an intensity equal to the complete melting point “Ec” is irradiated in order to maximize the size of the grains, very small grains are generated by the peak laser proceeding the complete melting point “Ec.” If the intensity of the peak exceeds the critical point “Ec,” the size of the grains generated sharply decreases as shown in “B” of FIG. 3.
However, as described above, if the peak is generated at the edges of the laser beam, the grain size is sharply changed into the very small area where the laser beam is irradiated. While the polysilicon having a large grain size is required to form a polysilicon TFT, it is also very important to form the polysilicon evenly. If the polysilicon is formed with grains having various sizes by using a laser beam having the peak, characteristics of the LCD device are deteriorated.